The present invention generally relates to methods and apparatus for flattening the gain spectra of optical amplifiers in dense wavelength division multiplexing (DWDM) communications systems, and more particularly, for flattening the spectral profile of the gain of erbium-doped fiber amplifiers (EDFAs).
High-speed fiber-optic communications networks are becoming increasingly popular for data transmission due to their high transmission bit-rate and high information carrying capabilities. The explosive growth of telecommunication and computer communications, especially in the area of Internet, has placed a rapidly expanding demand on national and international communications networks. This tremendous amount of worldwide data traffic volume requires fiber-optic communications networks having multi-gigabit transmission capacity with highly efficient cross-connect links.
To this end, in the field of fiber-optic technology, products have been developed for multi-carrier transmission over a single fiber, which multiplies the amount of information capacity over a single carrier system. Several individual data signals of different wavelengths may be assembled into a composite multi-channel signal that is transmitted on a single fiber, commonly referred to as wavelength division multiplexing (WDM). Accordingly, with WDM, multiple users are able to share a common fiber-optic link which realizes high throughput. To assemble the multi-channel signals, a multiplexing device (MUX) is employed at the transmitting end, which combines the multiple light-wave signals from several sources or channels of different wavelengths into the single composite signal.
In order to avoid cross-talk between channels, the center wavelengths of the signals must be properly spaced and the pass bands must be well defined. For example, the well-accepted industrial standard is a channel spacing of 100 GHz (0.8 nm in 1.55 μm window) centered at the ITU grid with each signal channel having a pass bandwidth of 0.3 nm at 0.5 dB down power level. The multiplexed signal is then transmitted on a single fiber-optic communications link. At the receiving end, a demultiplexing device (DEMUX) separates the composite signal received from the fiber link into their original channel signals, each of which is a single signal channel centered at the ITU grid.
Dense wavelength division multiplying (DWDM) technology dramatically increases the information-carrying capacity transmitted on a single carrier fiber. For example, a 40-channel 100 GHz DWDM system with a 10 Gb/s transmission rate can transmit 400 Gb/s data in the C-band (1528-1563 nm). The number of channels deployed in long-haul DWDM systems is rapidly increasing to now beyond 100 over the C-band and L-band (1575-1610 nm). The MUX and DEMUX devices, in particular those with high-count channels, can be combined with other fiber-optic components to create new-generation products, thereby intensifying the networks' functionality.
An important issue in long-haul DWDM communications systems is related to the attenuation of signal power due to the presence of insertion, distribution, and transmission losses. The launched signals gradually decay as they propagate along the optical fibers. To boost the signal power, fiber amplifiers are employed periodically to compensate for the power loss. However, not all channels are amplified by the same factor because the gain spectrum (or gain profile) of an optical amplifier is not uniform. For example, as seen in FIG. 1, the gain spectrum of an EDFA has asymmetrical twin peaks due to a luminescent spectrum caused by the fine structure of the energy levels. Because the gain spectrum is not flat, there exists power deviation between amplified signals.
In long-haul transmission systems, optical signals are transmitted through a multi-amplifier system, such that differences between optical signal powers are accumulated. However, it is essential that channel powers of the multi-channel optical signal be approximately at the same level for optimal performance of DWDM systems. In other words, the system should have no spectral ripple across the bandwidth of whole channels. Accordingly, there is a need for a technique and device for flattening the gain spectrum of optical amplifiers to reduce the undesirable non-uniformity of channel powers.
Several prior-art techniques have been developed to statically flatten the gain spectrum of EDFAs across the bandwidth of 30-40 nm. This family of devices is known as fixed gain flattening filters (GFF). The basic idea behind these devices is to fabricate an optical filter whose transmission function (loss spectrum) versus wavelength is proportional to the inverse of the gain spectrum of the optical amplifier. When the signal amplified by an optical amplifier passes through such a filter, the power in the flat spectral regions will be reduced with respect to the lowest power level across the whole wavelength range so that the resulting power becomes uniform. In one approach, an optical notch filter is incorporated within the length of an erbium-doped fiber amplifier. Careful choice of the filter characteristics via multi-layer coating and location makes it possible to enhance the amplifier gain performance at wavelengths around 1550 nm. An amplifier with 27-dB gain and 33-nm bandwidth can be produced.
In another approach, a combination of long-period fiber Bragg gratings are used to produce an optical filter whose transmission spectrum counteracts the EDFA gain non-uniformities. The gain flattening is achieved across a bandwidth exceeding 40 nm. Other prior art filters, such as Mach-Zehnder filters and etalon-type filters are also employed for this purpose. Yet still another approach is to use a dual-core fiber to provide a relatively flat gain from 1525 to 1555 nm for EDFAs.
These gain flattening filters are truly static devices in the sense that their transmission spectrum functions are fixed once the fabrication is completed. From the application point of view, it is impractical to apply a fixed spectral profile to optical amplifiers with different gain profiles. This is to say that the fixed gain flattening filters will lead to a large residual non-uniformity of gain spectrum though they may work well for some particular optical amplifiers. Recently, a costly dynamic gain flattener has been developed to dynamically equalize the uneven spectral distribution resulting from optical amplification.